The present invention relates to system and method for controlling a vehicle running speed to a preset target speed, and more specifically vehicle speed control system and method for a vehicle equipped with a continuously variable transmission.
A vehicle speed control system of one conventional example as shown in FIG. 12 for a vehicle equipped with a continuously variable transmission is arranged to control only the throttle opening when a vehicle speed deviation is smaller than a predetermined value, and to control only the transmission ratio when the vehicle speed deviation is greater than the predetermined value (as disclosed, for example, in Japanese Patent (Kokai) Publication 61(1986)-232927). This system does not control both inputs, the throttle opening and the transmission ratio, simultaneously.
In another example for a vehicle equipped with a continuously variable transmission, a control algorithm (as disclosed in Shigeyuki Morita, "Control Algorithm for DBW", Jidoshagijutsu gakkai-shi", Vol. 48, No. 10, 1994) is designed to control the throttle opening and the transmission ratio by finding a pair of engine torque value and transmission ratio value to achieve an optimum fuel consumption, among pairs of engine torque and transmission ratio to achieve a driving force required to maintain a preset command vehicle speed value. This control system achieves the command vehicle speed value by controlling both inputs, the throttle opening and the transmission ratio.